Beneath the iconic silhouette of Mount Fuji lies Narusawa Cave, a natural tunnel sculpted by the relentless flow of lava over millennia. This geological wonder, also known as a "lava tube," owes its existence to the significant eruption of Mount Fuji on its northwestern slope in 864 AD. This cataclysmic event blanketed the area with a vast lava field, which today is carpeted by the dense Aokigahara forest, famously referred to as the "Sea of Trees." Narusawa Cave stands as one of the region's three largest subterranean formations, alongside Vitri Fugaku Cave and Kazani Cave of Sai Lake.
Stretching approximately 150 meters in length and reaching a height of about 3.6 meters, the cave maintains a surprisingly consistent internal temperature, just over 3 degrees Celsius. This cool, stable environment is conducive to the formation of delicate stalactites and stalagmites, adding to the cave's ethereal beauty. The optimal period to witness these lava formations in their full glory is during the winter or early spring. During these months, the lava stalagmites can reach impressive dimensions, with thicknesses of up to 0.5 meters and heights nearing 3 meters.
Historically, Narusawa Cave served practical purposes beyond its natural allure. It was employed as a natural refrigerator for storing ice, particularly during the Edo period, when ice harvested from its depths was transported to the shogunate in Edo. In the early 1900s, before the advent of electric refrigeration, the ice produced here was vital for cooling. The cave also played a role in the processing of silkworm cocoons during the colder seasons, showcasing its multifaceted utility.
Within the cave's depths, the remnants of ancient trees, engulfed by lava flows during the 864 AD eruption, offer a poignant glimpse into the past. Narusawa Cave's eastern expanse extends for 800 meters, leading towards Vitri Fugaku Cave, which delves much deeper into the earth. These lava formations and their unusual columnar structures are a testament to the powerful volcanic forces that shaped this landscape. Unlike Narusawa Cave, Vitru Cave's solid basalt walls absorb sound, creating a unique acoustic environment where echoes are virtually nonexistent, a stark contrast to the sound-reflecting formations found in Narusawa Cave due to its icy deposits.